To date, carbon fibers have been used in various composite materials because of their good mechanical properties and high electric conductivity.
Recently, higher functionalities have come to be required for various materials. Additives which can improve physical properties, such as electric, mechanical, or thermal properties, of a matrix comprised of solid materials, such as resin, ceramics, and metal, without damaging the characteristics of the matrix have been sought after. Additionally, additives which can improve physical properties of liquids, such as fuels, oil, and lubricants have also been sought.
Incidentally, fine carbon fibers, such as carbon nano structures exemplified by the carbon nanotube (hereinafter, referred to also as “CNT”), have been attracting public attention in various fields.
The graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered rings whose structures can bring about specific electrical properties, as well as chemically, mechanically, and thermally stable properties. As long as such fine carbon fibers can retain such properties upon combining and dispersing into solid materials, including various resins, ceramics, metals, etc., or into liquid materials, including fuels, lubricant agents, etc., their usefulness as additives for improving material properties can be expected.
On the other hand, however, such fine carbon fibers unfortunately show an aggregate state even just after their synthesis. When these aggregates are used as-is, the fine carbon fibers would be poorly dispersed, and thus the product obtained would not benefit from the desirable properties of the nano structures. Accordingly, given a desired property such as electric conductivity for a matrix such as resin, it is necessary that the fine carbon fibers would be added in a large amount.
Patent Literature 1 discloses a resin composition comprising aggregates wherein each of the aggregate is composed of mutually entangled carbon fibrils having 3.5-70 nm in diameter, and wherein the aggregates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. It is noted that the numeric data such as the maximum diameter, diameter, etc., for the carbon fibril aggregates are those measured prior to combining with a resin, as is clear from the descriptions in the examples and other parts of the Patent Literature 1. Patent Literature 2 discloses a composite material where a carbon fibrous material is added to the matrix, the carbon fibrous material mainly comprising aggregates each of which is composed of carbon fibers having 50-5000 nm in diameter, the mutual contacting points among the carbon fibers being fixed with carbonized carbonaceous substance, and each aggregates having a size of 5 μm-500 μm. In the Patent Literature 2, the numeric data such as the size of aggregate, etc., are those measured prior to the combining into resin, too.
Using carbon fiber aggregates such as described above, it is expected that the dispersibility of carbon nano structures within a resin matrix will improve to a certain degree as compared to that of using bigger lumps of carbon fibers. The aggregates prepared by dispersing carbon fibrils under a certain shearing force, such as in a vibrating ball mill or the like according to the Patent Literature 1, however, have relatively high bulk densities. Thus, they do not fulfill the need for ideal additives that is capable of improving various characteristics, such as electric conductivity, of a matrix effectively at minuscule dosages.
With respect to the carbon fibrous structure disclosed in the Patent Literature 2, it is necessary to provide an additional step for fixing carbon fibers at their mutual contacting points after synthesis of the carbon fibers, and thus the efficiency of manufacturing becomes worse. Further, since the carbon fibrous structure is manufactured by heating carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression molding after synthesis of the carbon fibers, and wherein fixing of fibers at the contacting points is done by carbonization of organic residues primarily attached to the surface of the carbon fibers, or carbonization of an organic compound additionally added as a binder, the affixing forces at the contacting points are weak. In addition, the electrical properties of the carbon fibrous structures per se are not well, although a certain degree of improvement in the electrical properties would be expected as compared with the case of pulverized monofibrous carbon fibers. Thus, when these carbon fibrous structures are added to a matrix such as a resin, the carbon fibers fixed at the contacting points are easily detached from each other, and the carbon fibrous structures are no longer maintained in the matrix. Therefore, it is not possible to construct preferable conductive paths in a matrix such that good electrical properties may be conferred on the matrix by a small additive amount of the fibrous structures. Furthermore, when a binder is added to promote fixing and carbonization at the contacting points, fibers in the obtained fibrous structures would have large diameters and inferior surface characteristics because the added binder is attached to the whole surface area of the fibers rather than to a limited area on the contacting points.    Patent Literature 1: Japanese patent No. 2862578    Patent Literature 2: JP 2004-119386 A